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Abstract: SA-PO205

Burosumab: An Option for Adults With Symptomatic X-Linked Hypophosphatemia

Session Information

Category: Bone and Mineral Metabolism

  • 402 Bone and Mineral Metabolism: Clinical

Authors

  • Belal, Amer Ashaab, University of Florida College of Medicine, Gainesville, Florida, United States
  • Ruchi, Rupam, University of Florida College of Medicine, Gainesville, Florida, United States
Introduction

X-linked hypophosphatemia (XLH) is a dominant disorder caused by an inactivating mutation in the PHEX gene. This increases fibroblast growth factor 23 (FGF23) that subsequently acts to inhibit phosphate reabsorption in the proximal renal tubules. While traditional treatment with phosphate replacement and calcitriol meets some goals, side effects such as nephrocalcinosis limit their use. Thus in 2018 the FDA approved Burosumab as a human anti-FGF23 monoclonal antibody for adults.

Case Description

We discuss the case of a 39-year-old woman with history of hypophosphatemic vitamin D resistant rickets diagnosed at 18 months. She has been followed by adult nephrology since age 19. At that time, she had lab work notable for 1, 25 Vitamin D level 61 pg/mL and PTH 134 pg/mL. Her urinary phosphate was inappropriately high at 0.6 g/24 hours despite hypophosphatemia of 1.8 mg/dL. As a child she was on phosphate supplementation but this was stopped due to nephrocalcinosis. She was maintained on dietary phosphate and calcitriol for the past twenty years with stable severe hypophosphatemia resulting in profound fatigue. Recently she completed genetic testing for PHEX gene mutation and FGF23[RR1] . Her FGF23 was normal at 140 RU/mL and her PHEX gene mutation was heterozygous positive for the pathogenic c.1715G>A (p.Gly572Asp) variant. She has a symptomatic son and a healthy daughter. Given her symptoms of fatigue and bone pain in setting of hypophosphatemia, she was started on Burosumab at 60 mg q4 weeks. Her phosphate improved to 3.2 mg/dL from 1.5 mg/dL with increase in total calcium to 10.3 mg/dL and ionized calcium 1.41 mmol/L. Her dose was decreased to 40 mg monthly with repeat labs showing normal calcium and phosphorous of 2.6 mg/dL with improved symptoms. On this dose her phosphate levels remained between 2.3-3.2 mg/dL and calcium levels 9.9-10.2 mg/dL until she was unfortunately lost to follow up resulting in need for cessation of Burosumab therapy with return of her symptoms of fatigue, bone/joint pain, and severe hypophosphatemia of 1.3 mg/dL. She has now been restarted on Burosumab therapy

Discussion

This case is emblematic of the benefits of Burosumab therapy for adults with XLH. In keeping with clinical trials, treatment with Burosumab resulted in increased activity levels and overall quality of life along with improved phosphorus levels. Thus, we hold that it should be offered to symptomatic adults with XLH